In a mass spectrometer, MS/MS analysis in the following procedure is often performed in which ions of a specific mass are selected from ions generated at an ion source, the ions are dissociated, and a mass of fragment ions is analyzed, so that the detailed structure of a sample is identified. For example, in the case of a mass spectrometer where all of an ion transport unit (Q0), a first ion selection unit (Q1), an ion dissociation unit (Q2), and a second ion selection unit (Q3) are configured of a multipole rod electrode (typically, a quadrupole rod electrode), ions generated in an ion source are efficiently passed through Q0 by applying a radio frequency (RF) voltage to the multipole rod electrode of Q0, and introduced into Q1. Q1 is called a quadrupole mass filter (QMF) because Q1 can pass only ions of a specific mass among the introduced ions by applying an RF voltage and a direct current (DC) voltage to its multipole rod electrode. The specific ions selected and separated at Q1 are introduced into Q2. Q2 is called a collision cell because Q2 includes a function (CID: Collision Induced Dissociation) that dissociates ions by causing ions to collide against a neutral gas (such as nitrogen, helium, and argon) in the atmosphere of Q2 while passing ions by applying an RF voltage to the multipole rod electrode. The ions dissociated at Q2 are introduced into Q3. Q3 is also called a QMF because Q3 can pass ions while separating the introduced ions according to masses by applying an RF voltage and a DC voltage to the multipole rod electrode as similar to Q1. The ions separated at Q3 are ejected from an outlet according to masses, and detected at a detector.
Since general ion dissociation at Q2 is performed by causing ions to collide against a neutral gas, the ions introduced into Q2 repeat collision to slow the rate of travel, and the time of flight in Q2 is prolonged. Although depending on the length of Q2 or ion masses, generally, it takes a few milliseconds to pass ions through Q2. Therefore, it is difficult to improve the throughput of analysis.
Patent Literature 1 proposes various methods in order to shorten the ion time of flight in Q2. The detail is shown below.    (1) A multipole rod electrode is divided in the axial direction, and different DC offset voltages are applied to the divided electrodes to form an axial electric field, and then ions are accelerated and passed in the axial direction with the electric field.    (2) The multipole rod electrode is configured of a rod electrode in a tapered shape to form an axial electric field, and ions are accelerated and passed in the axial direction with the electric field.    (3) The rod electrodes of the multipole rod electrode are disposed obliquely to form an axial electric field, and ions are accelerated and passed in the axial direction with the electric field.    (4) An electrode to form an axial electric field is disposed at a position in a gap between the rod electrodes of the multipole rod electrode, and ions are accelerated and passed in the axial direction with the electric field.    (5) The multipole rod electrode is configured of a rod electrode having a resistor coating, and a potential difference is applied across the both ends of the rod electrode to form an axial electric field, and ions are accelerated and passed in the axial direction with the electric field.